Even though these numerical differences look quite large, they seem to be purely due to a change in calculation precision.

I've tried doing the interpolation calculations myself "by hand". If I use float32 all the way through (even though the grid coordinates are specified as float64) then I end up with approx. 403.770. If I use float64 I end up with approx. 403.764.

Arguably the latter is more accurate. So I'm OK with this change.